ture, as does debate regarding the use of combined schedules of IPV and OPV (Institute of Medicine, 1977, 1988).
The American Academy of Pediatrics and the Advisory Committee on Immunization Practices recommend that OPV be administered at ages 2, 4, and 15 months and again at ages 4 to 6 years. If IPV is used, it should be given according to the same schedule as OPV.
Each of the three immunologically distinct types of poliovirus—types 1, 2, and 3—can cause paralytic disease. Both IPV and OPV stimulate immune responses against all three types of virus. OPV induces gastrointestinal mucosal immunity to a greater degree than IPV. The enhanced-potency IPV used today in the United States produces a humoral antibody response superior to that of OPV (Onorato et al., 1991). It is not clear whether either OPV or IPV confers lifelong immunity (Nishio et al., 1984). The mechanism of attenuation of the neurovirulence and that of occasional reversion to neurovirulence have been described in detail in a recent review (Racaniello, 1992). Recipients of OPV shed the virus in their feces, and contacts exposed to the virus can become infected.
Infection with poliovirus can take several forms: inapparent infection, mild illness, aseptic meningitis (nonparalytic poliomyelitis), and paralytic poliomyelitis. Approximately 4 to 8 percent of all wild-type poliovirus infections result in nonparalytic polio disease. This manifests as fever, malaise, headache, nausea, stiffness of the neck and back, and meningeal signs. Approximately 1 percent of infections results in paralytic disease. In paralytic poliomyelitis, the virus invades the central nervous system, replicating in motor neurons within the anterior horn of the spinal cord, in the brainstem, and in the motor cortex. When viral replication destroys sufficient numbers of neurons, paralysis occurs (Racaniello, 1992). The illness begins with a headache, fever, and stiff neck; this is followed by paralysis of the voluntary muscles previously controlled by the destroyed neurons of either the spinal cord or the brainstem. The muscle paralysis is usually asymmetrical. The spinal fluid contains an increased number of lymphocytes, the protein concentration is elevated, and the glucose concentration is normal.